In paving operations, three main practices are employed to achieve thorough mixing of bitumen and aggregate:
(1) mixing of free flowing heated asphalt (asphalt cement) with pre-dried aggregate,
(2) mixing pre-dried aggregate with asphalt diluted with a hydrocarbon solvent (cutback asphalt, cutter stock) at ambient or slightly elevated temperatures, and
(3) mixing aggregate with asphalt emulsions, e.g., oil in water emulsions, obtained by vigorous agitation of asphalt and water in the presence of an emulsifying agent.
The escalating costs of energy and hydrocarbon solvents coupled with a heightened environmental awareness have stimulated increases in the use of emulsified asphalts in the road paving industry. The type of emulsifier employed is determined by the desired application of the asphalt emulsion. For rapid set emulsions (mainly used for chip sealing) sodium soaps of tail oil are commonly utilized. For medium set emulsions (applied in cold mixes of virgin aggregate or reclaimed asphalt pavement) higher concentrations of tall oil or modified tall oil soaps are generally being used with and without the addition of moderate amounts of hydrocarbon solvent. Slow set emulsions with good mix stability in the presence of fine graded aggregate are usually based on vinsol (a by-product of the wood rosin manufacture), on fortified tall oil rosin in combination with kraft lignin or lignosulfonates, and combinations of kraft lignin or lignosulfonates with nonionic emulsifiers from the class of ethoxylated alkylphenols, ethoxylated linear or branched fatty alcohols and of ethylene oxide-propylene oxide-block co-polymers. In anionic emulsions the asphalt droplets are stabilized by anionic surfactants (wherein their negatively-charged surface migrates to the anode when an electric field is applied).
In the case of rapid set emulsions (mainly used for repair work of old wearing courses) the emulsion is applied on the existing surface and aggregate is spread on top. After the water of the emulsion has evaporated, an intimate matrix of asphalt and stone with good load beating capacity is formed. The road can be re-opened to traffic shortly after application of the seal. Medium set emulsions are commonly being mixed with aggregate in a pug mill prior to being used in road construction. The incorporation of solvent allows the mixes to be stock piled prior to use. The mixes are prepared in central mixing plants and transported to the job sites or are generated "in-place". Slow set emulsions are being applied where good penetration and wetting is necessary. Mixes with high loadings of fines, base stabilization and tack coat are the main applications.
Anionic emulsions are taught by Mertens in U.S. Pat. No. 3,062,829 to be prepared via the use of alkali hydroxide which saponify the surface active acids naturally occurring in asphalt. These emulsions contain high molecular weight polyamides (Versene) as viscosity reducers and adhesion promoters. In U.S. Pat. No. 3,108,971 to Mertens anionic emulsions of the same type are improved with the addition of alkanol amines lacking lipophilic characteristics. Lignin amines are taught by Borgfeldt in U.S. Pat. No. 3,123,569. Quick setting emulsions obtained from highly acidic asphalts using lithium hydroxide are disclosed by Mertens in U.S. Pat. No. 3,240,716. Montgomery and Pitchford teach the alkali metal salts of complex polynuclear aromatic polycarboxylic acids as anionic emulsifiers in U.S. Pat. No. 3,344,082. Heinz in U.S. Pat. No. 3,006,860 employs alkali metal soaps of higher fatty acids such as those found in tall oil. In U.S. Pat. Nos. 3,956,002 and 4,088,505 Moorer teaches anionic emulsifiers consisting of alkali lignin or oxygenated alkali lignin, an ethylene oxide adduct of alkylphenol and up to 10% by weight of sodium borate. Detroit describes in U.S. Pat. No. 4,293,459 combinations of partially desulfonated oxygenated lignosulfonates and nonionic surfactants. Schilling et al. disclose the alkali soaps of maleated or fumarated tall oil fatty acids or rosin, of DIACID.RTM. 1550 and of sulfonated tall oil fatty acid as emulsifiers for anionic high float emulsions in U.S. Pat. No. 4,676,927 and the use of carboxyethylated modified tall oil amidoamines as emulsifiers for anionic slurry seals in U.S. Pat. No. 4,561,901. Ferm in U.S. Pat. No. 3,740,344 teaches the preparation of quick set anionic slurry seal compositions by applying a combination of aryl alkyl sulfonates and ethylene oxide adducts of alkyl phenols and of fatty alcohols. Schreuders in U.S. Pat. No. 3,615,796 teaches the use of petroleum sulfonates. A combination of sodium lignate or lignosulfonate and saponified tall oil or rosin is disclosed in U.S. Pat. No. 3,594,201 by Sommer and Evans. In U.S. Pat. No. 3,350,321 Conn describes the use of alkyl or alkoxy alkyl phosphoric acid salts as emulsifiers for asphalt.
Anionic emulsions are generally prepared at emulsifier concentrations of 0.2-10.0% based on 100% activity, preferentially at 0.2 to 2.0%. The pH range is 7 to 14, preferentially at 10 to 12 in the case of tall oil and rosin soaps. The advantage of anionic emulsions lies in the relatively low cost of tall oil based emulsifiers. The disadvantage is the low bond strength of asphalt to aggregate once the emulsion has dried and formed a film of asphalt on the surface of the aggregate. As most of the aggregates are negatively charged, the electrostatic repulsion between the negatively charged asphalt and the negatively charged stones causes inferior adhesion. Highly acidic aggregates such as quartzite, granite, rhyolite and many of the sedimentary, metamorphic and igneous rocks are considered responsible for the existing bitumen-stripping problem. This problem is also encountered in hot mix applications and when cut back asphalts are being used.
The quality of the road surface is generally dependent upon the strength of the bonds between the asphalt and the aggregate after curing of the composition. Poor service performance is due to poor adhesion, which results in asphalt stripping off the aggregate surface. Asphalt compositions also have relatively poor adhesion to aggregate in the presence of water. Since the aggregate is preferentially wetted by water, the eventual penetration of water into the composition reaches the aggregate and interferes with the bond between aggregate and asphalt. The result of this stripping is flaked pavement and the formation of pot holes.
To reduce water-induced debonding of asphalt from the stone surface, in many cases surface-active amines or diamines are added to the asphalt. Generally, anti-stripping agents or adhesion promoters are introduced into the asphalt prior to the asphalt being mixed with the aggregate. In the case of anionic asphalt emulsions it is advantageous to add the additive to the emulsion to prevent degradation at the high pH values. The patent literature sets forth a large number of compounds which can be used to improve adhesion of asphalt to aggregate. These include ethylene oxide condensates of long chain alkyl triamines (U.S. Pat. No. 3,615,797), alkoxylated amines and their salts (U.S. Pat. No. 3,347,690), and reaction products of ozonized unsaturated fatty acids with polyalkylene amines (U.S. Pat. Nos. 3,246,008 and 3,245,451). Other additives are based on fatty carboxylic chromites (U.S. Pat. No. 3,963,509), on combinations of epoxy resins and onium borates (U.S. Pat. No. 3,947,395), on tall oil alkanol amines and amido amines (U.S. Pat. Nos. 2,679,462 and 4,806,166), on fatty ether amines in combination with alkanol amines (U.S. Pat. No. 3,928,061), and on fatty acid amido amine soaps (U.S. Pat. Nos. 2,426,220, 2,891,872 and 3,230,104). Aminoalkyl polyalkoxysilanes are disclosed in U.S. Pat. No. 3,861,933; and condensation products of amines, polyamines, and amides with formaldehyde are taught in U.S. Pat. No. 4,639,273. Mannich reaction products of polyamines with formaldehyde and alkylphenols are described in U.S. Pat. No. 4,789,402, and ethoxylated hexamethylenediamines and their derivatives are taught in European Patent Application No. 0 077 632 (82305420.0). Fatty primary, secondary and tertiary amines and imidazolines, their reaction products with various acids (including fatty acids), metal soaps, and several other compounds including rosin reaction products are described in U.S. Pat. No. 3,868,263.
One relatively inexpensive class of adhesion promoters which have shown promise for use in hot mix and in cut back asphalts are tall oil-based polyethylene amine condensation products. However, a major problem exists with such adhesion promoters in that their adhesion efficiencies are not high enough to obtain satisfactory results when they are utilized in anionic emulsions. It is, therefore, the object of this invention to solve this problem by disclosing an improved method for enhancing adhesion between asphalt and aggregate in anionic bituminous emulsions.